1. Field of the Invention
The present invention relates to a solution film-forming method for use in manufacturing films for photosensitive materials and optical applications, and a protective film of a polarizing plate, an optical functional film, a polarizing plate and a liquid crystal display device constituted by using the polymer film formed by the solution film-forming method.
2. Description of the Related Art
In recent years, in transparent films, the demand for optical materials such as a protective film of a polarizing plate for a liquid crystal display device, an optical compensation film such as a phase difference plate, a transparent substrate and an optical filter has grown in addition to the conventional demand for a photo support or an overhead transparency, which has been increasing the severity of quality demands for the transparent film.
In particular, a plastic film for optical materials such as a protective film of a polarizing plate needs to have high transparency, low optical anisotropy, planarity, easiness of surface treatment, high durability and scratch resistance.
Plastic films with these characteristic properties include a cellulose ester, a norbornene resin, an acrylic resin, a polyacetate resin and a polycarbonate resin, and cellulose ester is mainly used from the view points such as productivity and a material price. In particular, cellulose triacetate (TAC) is advantageously used for its high transparency, low optical anisotropy and low retardation.
A cellulose acetate film is generally manufactured by a solution film-forming method including the steps of casting a polymer liquid (dope), which has a polymer dissolved in an organic solvent, on a support comprising a surface-cooled drum or an endless band from a casting die, cooling and drying it on the support, peeling a solidified film from the support and drying it while conveying it with rolls or tenters.
A method for facilitating gelation of the dope to shorten the time of the peeling from the support by adding a poor solvent to a main solvent has been proposed in order to increase the production speed of the film by this solution film-forming method and improve the production efficiency (such as U.S. Pat. No. 2,607,704 class).
However, since the boiling point of the poor solvent to be used is generally high, it takes time to dry the film after peeling and the total production time of the film will remain unchanged. There may be a method for casting a dope having a higher concentration on the support in order to decrease the load for drying. Higher concentration of the dope, however, could generate melt fracture, which can extremely reduce the quality of the surface condition of the film.
Melt fracture is a phenomenon in which irregular unevenness is formed on the film surface or gloss of the surface is lost. The cause of the melt fracture is assumed that a large shear stress of a dope flow can disturb the flow state of the dope at a flow inlet to a die to prevent the flow from forming a parallel flow, or that a dead space can be formed in a dope flow path inside the die when relaxation of the internal stress takes time due to a too-high dope viscosity.
A method for preventing occurrence of the melt fracture is disclosed in Japanese Patent Publication No. 62-43846 in which both high- and low-viscosity solutions are simultaneously extruded from a die slit in such a manner that the high-viscosity polymer solution is enveloped by the low-viscosity solution to reduce the shear stress inside the die slit to a threshold value or below.
On the other hand, when the content of an organic solvent at the peeling of the film from the support exceeds a specific value, unstable conveying such as a sag or flapping of a peeled film from the support in conveying means could occur as the film is soft, resulting in the problem of failures of the surface condition such as scratches, knicks and wrinkles.
To solve this problem, the film needs to be conveyed under an appropriate tension. However, if the content of an organic solvent exceeds a specific value, the tension could shrink the film. Therefore, Japanese Patent Laid-Open No. 5-185445 discloses a method for applying tension to the film while it is subjected to dry air within a certain temperature range for a certain period of time after the film is peeled from the support.
On the other hand, in a drying step for drying a film while conveying it with rolls or tenters, there may be a problem of occurrence of continuous wrinkles (stringiness) in the conveying direction of the film if the drying temperature is high in order to increase the production speed of the film.
In order to improve the stringiness formed in the conveying direction of the film, there may be a method for providing a planarity-improving step comprising a heating step by a heating roll and a cooling step by a cooling roll as a downstream step of the drying step. However, a plasticizer in the film precipitated on the heating roll can adhere to the film as a thin film or form a spot pattern on the film.
Japanese Patent Laid-Open No. 9-225953 discloses a method for imparting a cationic or anionic polymer to avoid precipitation of the plasticizer or formation of the spot pattern when applying a film upstream the planarity-improving step.
However, streak defects caused by a nonuniform thickness in the casting direction can be formed also by the conditions when casting the dope on a support from a die.
Japanese Patent Laid-Open No. 9-207194 and No. 9-207195 disclose methods for preventing the streak defects by R-processing of the tip of die lips. Furthermore, Japanese Patent Laid-Open No. 2001-71338 discloses a method for preventing the streak defects by limiting the stretching stress applied to the ribbon-shaped dope at the tip of the die lips or the pin angle of the ribbon-shaped dope.
In addition, when an additive liquid such as methanol or butanol is mixed with a liquid to be added such as a cellulose ester or an organic solvent, uniform dispersion and mixing of the additive liquid with the liquid to be added depend on the viscosity ratio, flow volume ratio and the like of these fluids. Thus, the nonuniform thickness of the film is influenced also by the dispersibility of the mixture.
Japanese Patent Laid-Open No. 2001-113525 discloses a method for adjusting a polymer solution to uniformly disperse and mix an additive liquid with a liquid to be added by restraining generation of pulsation in the additive liquid by bringing the ratio of flow rate of the additive liquid and the liquid to be added at a converging section within a specific range.
By using these methods, it is possible to restrain the thickness distribution of a cellulose acetate film in a lateral direction and a longitudinal direction at least 0.5% or below.
However, a radical improvement of flowability of the polymer liquid cast from a casting die is difficult only by the improvement of the structure of the casting die such as disclosed in Japanese Patent Laid-Open No. 9-207194, No. 9-207195, and No. 2001-71338. Furthermore, since the polymer solution containing a cellulose ester and the like is fundamentally a viscoelastic body and has elastic properties, it is necessary to take dynamic viscosity and dynamic modulus into account. In addition, for the films for optical applications requiring homogeneous control of the surface condition such as a polarizing plate for a liquid crystal display device and the like, a phase difference plate and an optical compensation film, the demands for the quality of the surface condition tend to be increasingly severe.